620 J. CHEM. RESEARCH (S), 1999
J. Chem. Research (S),
1999, 620^621y
Deoxygenation Reaction of Phenyl Nitronyl
Nitroxides with the Strong Acceptors TCNQF
and TCNQ
Shin'ichi Nakatsuji,* Atsushi Takai, Takeo Ojima and
Hiroyuki Anzai
Department of Material Science, Faculty of Science, Himeji Institute of Technology, Kanaji
1475-2, Kamigori, Hyogo 678-1297, Japan
The reaction of certain phenyl nitronyl nitroxide derivatives with strong acceptors such as TCNQF4 or TCNQ in
appropriate solvents give the corresponding imine nitroxide derivatives by an anomalous deoxygenation reaction
with acceptors in a selective manner.
Nitronyl nitroxide radicals (4,4,5,5-tetramethyimidazoline-
1-oxyl-3-oxides) are a well-known class of stable radicals
used as spin probes for biological studies and/or are
promising substrates for organomagnetic materials and a
number of derivatives has been prepared for both purposes.1
It is also known that imine nitroxide radicals can generally
be obtained from the corresponding nitronyl nitroxides
by treatment with triphenylphosphine or nitrous acid as
deoxygenating agents.2 Akaike et al.3 and Joseph et al.4
have independently reported interesting deoxygenation
reactions of some nitronyl nitroxide derivatives with ÁNO
produced in biological systems and Sugimoto et al. have
reported in a more recent publication that certain nitronyl
nitroxides could be easily reduced to the corresponding
imine nitroxides simply by standing the substrates in
n-heptane or acetone solution at room temperature.5
TCNQ (7,7,8,8-tetracyanoquinodimethane) and TCNQF4
(2,3,5,6-tetra£uoro-7,7,8,8-tetracyanoquinodimethane) are
classical, strong organic acceptors6 with cyanide function-
ality, and are used mainly as the components of CT com-
plexes with appropriate donors to a¡ord functional
materials such as organic conductors.
During the course of our studies on the development of
new organomagnetic materials based on nitroxide radicals,7
we found recently that some TEMPO radicals8 are able
to form CT complexes with appropriate acceptors such
as TCNQF4 or DDQ (2,3-dichloro-5,6-dicyano-1,4-
benzoquinone). On the other hand, similar attempts to pre-
pare CT complexes from nitronyl nitroxides with TCNQF4
or TCNQ in solution were fruitless, but imine nitroxide
radicals were found to be obtained in place of CT com-
plexes. Here we report an anomalous deoxygenation
reaction of various phenyl nitronyl nitroxides by using
TCNQF4 or TCNQ to prepare imine nitroxide radicals
which are also signi¢cant in biological9 as well as materials
science.7
We then investigated the scope and limitation of the
deoxygenation reaction by applying it to various nitronyl
nitroxide radicals (Scheme 1). The deoxgenation reaction
of 1a proceeded smoothly in acetonitrile, tetrahydrofuran
or ethanol in 2^5 min and rather gradually in acetone,
a¡ording 1b as the sole isolated product after column
chromatography on Al2O3, but no reaction was observed
in nonpolar solvents such as dichloromethane or benzene.
As shown in Table 1, neither the reaction time nor the vari-
ation of solvent employed seriously a¡ected the yield of 1b.
Moreover, the weaker acceptor TCNQ was found to be
similarly e¡ective in the reaction, although the reaction with
TCNQ proceeds rather slowly and hence needs more
reaction time. On the contrary, no deoxygenation could
be observed when 1a was treated with DDQ, which suggests
that the dicyanoalkene unit plays an indispensable role in
the reaction.
TCNQ(F4)
O2 adduct of TCNQ(F4)
O
O
N
N
N
R
R
N
–
+
O
Phenyl nitronyl
nitroxide radicals
Phenyl imino
nitroxide radicals
1a R = H
1b R = H
2a R = 4-F
3a R = 4-NO2
4a R = 2-MeO
5a R = 2-F
6a R = 2-Cl
2b R = 4-F
3b R = 4-NO2
4b R = 2-MeO
5b R = 2-F
6b R = 2-Cl
Scheme 1
The reaction could also be applied to other phenyl
nitronyl nitroxide derivatives (2a^6a)12 with substituents
in o- or p-positions to form the corresponding imino
nitroxides 2b^6b as shown in Table 1. Thus, although
the isolated yields were not so high in each case, the
reaction was found to be applicable to various phenyl
nitronyl nitroxide derivatives with electron-donating or elec-
tron-withdrawing groups in the o- or p-positions of the rad-
ical substituent. It is to be noted that each reaction was
found to be fast and that the radicals could be readily
isolated without tedious separation although the yields were
moderate, with one being comparable to conventional
methods.3 However, no imino nitroxide derivatives, but
rather uncharacterizable mixtures of products, could be
isolated when the same reaction was applied to ethyl
nitronyl nitroxide.
When an acetonitrile solution of phenyl nitronyl nitroxide
1a10 was treated with an equimolecular amount of
TCNQF4, imine nitroxide 1b was found to be formed in
place of the corresponding CT complex, as revealed by
FAB-MS as well as EPR measurements. It appeared to
be interesting that reductive deoxygenation took place in
solution by using a strong acceptor, i.e. a strong oxidative
reagent, while Sugawara et al. obtained a DDQ complex
from dimethylaminonitronyl nitroxide by mixing each
component in the solid state.11
* To receive any correspondence (e-mail: nakatuji@sci.himeji-tech.
ac.jp).
This is a Short Paper as de¢ned in the Instructions for Authors,
Section 5.0 [see J. Chem. Research (S), 1999, Issue 1]; there is
therefore no corresponding material in J. Chem. Research (M).
In summary, we have found
a new deoxygenation
reaction for phenyl nitronyl nitroxide radicals by using
strong acceptors such as TCNQF4 or TCNQ in appropriate